Lift Control of Adaptive Wind Turbine Blades with Bend-Twist Coupling
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A major challenge in the development of wind turbines comes with designing the blades so that they are robust, low weight, cost effective and highly efficient. As the industry develops and as more turbines are built in offshore locations, larger rotor diameters will be implemented in an effort to reduce the cost of energy. This thesis investigates the potential for implementing passively adaptive blades with bend-twist coupling into multimegawatt horizontal axis wind turbine systems to save costs by decreasing loads. This thesis is based on a compendium of research papers but also includes an introduction to the topic and provides background information on blade materials/structure design, design with respect to failure criteria, mechanics of composite materials and a literature study of previous research performed on bend-twist coupled blades. The six research papers that were completed as part of this Ph.D. are included in the appendix. The key results from these papers are summarized in the final chapters of the thesis and presented with concluding remarks and suggestions for future research topics. Bend-twist coupling was examined through use of unbalanced composite laminates created with glass and carbon fiber plies. 3-dimensional models of large scale blades were designed and subjected to finite element simulations to observe the influence of bend-twist coupling on the structural failure modes of the blades. Furthermore, an experimental study was performed on lab-scale specimens to determine the fatigue failure modes and fatigue resistance of these materials in comparison to common materials for wind turbine blades, i.e. glass fiber. Parametric studies were performed considering the magnitude of bend-twist coupling, the achieved induced twist, the fiber stress, in-plane shear stress, tip deflection, buckling, fatigue and natural frequencies of the blade designs to further validate this technology. The various failure modes of the adaptive blades were dependent on the amount of bendtwist coupling and the blade stiffnesses, whereas the critical mode changed based on the definition of the composite layup. The primary results were that bend-twist coupled blades provided opportunities for significant load reduction for multi-megawatt wind turbines, and furthermore that blade size had negligible effects on the load reduction capability. In addition, a number of possible laminate designs were found to satisfy all design criteria while producing substantial amounts of blade twist. The in-plane strength criterion set an upper limit for the magnitude of induced twist, whereas the tip deflection and buckling criteria were the most critical from a mass (material usage) perspective. Optimization techniques to simultaneously maximize the induced twist and resistance to buckling allowed for reductions in blade weight.